Piece for example about unpleasant material (5)
I believe the answer is:
gunge
'unpleasant material' is the definition.
The definition suggests a singular noun which matches the answer.
'piece for example about' is the wordplay.
'piece' becomes 'gun' (slang term for a gun).
'for example' becomes 'e.g.' (short for 'exempli gratia', 'for example' in Latin).
'about' shows that the letters should be reversed in order.
'eg' reversed gives 'ge'.
'gun'+'ge'='GUNGE'
Can you help me to learn more?
(Other definitions for gunge that I've seen before include "This makes things sticky" , "Nasty and sticky stuff" , "Congealed matter" , "Unpleasant sticky material" , "Dirty substance" .)